1. Field of the Invention
The present invention relates to sensor devices, and in particular, to methods and structures for reducing current leakage from the sensor device.
2. Description of the Prior Art
Sensor devices are commonly used in a wide variety of reading apparatus such as scanners, facsimile machines, copying machines, digital cameras, PC cameras, video phones, video conferencing apparatus, door phones, security systems and the like. There are two types of sensor devices. One is a lineartype and the other is an array-type. Each sensor device receives light that has been reflected from an object that is being read. The light that is received by the sensor device can have different intensities depending on the object that is being read. The different light intensities produce different electrical charges at the sensor device, which stores these charges in a capacitor. The electrical charge at the capacitor is then provided to a switching circuit that provides all the electrical signals from the array of sensor devices to a processor for processing and output.
FIG. 1 illustrates the structure of a conventional sensor device 10. The conventional sensor device 10 has a substrate 11 (such as a P-well, N-well, P-substrate or N-substrate) and a field oxide layer 12 that is used to isolate the sensor device 10 from adjacent sensor devices. After the field oxide layer 12 has been formed, impurity diffusion or ion implantation methods that are well known in the art are used to introduce impurities (e.g., phosphorus or boron ions, among others) into the substrate 11 from a diffusion machine or ion implanter, respectively, to form a sensor 13 adjacent the field oxide layer 12. Sensor 13 can also be considered to be a junction capacitor.
Field oxide layer 12 is made in a humid and oxygen-rich environment using a wet oxidation method as is well-known in the art. Unfortunately, liquid and oxygen gas have a lateral diffusion effect on the edges of sensor 13, thereby producing a slowly tapering oxide 14 that wedges itself into the sensor 13 area. This wedge is also known as a "bird's beak". Because of the stress experienced during the oxidation that takes place during the conventional manufacturing process for the CMOS integrated circuit (IC), crystal defects are often generated at the wedges or bird's beak 14. The stress is caused by the different pressures of two different materials, in which each has a different density.
The bird's beak 14 adjacent to the junction edge of the sensor 13 is susceptible to damage during the subsequent manufacturing steps for the CMOS IC. For example, plasma damage can be caused by dry etching, and implantation damage can be caused by ion implantation. If the damaged junction edge cannot be fully recovered in the subsequent IC processing steps, then a current leakage path may be created from the junction capacitor to the substrate 11. When the leakage is large, it is impossible to maintain the voltage level stored in the capacitor, so that when the sensor device 10 performs its read-out operation, the sensor 13 may produce a "white pixel" or "hot line" or "dead pixel" or "dead line".
A "white pixel" is a pixel output that is always high because there is no current output (i.e., no charge at the capacitor), so that it would always appear to be a white spot (i.e., zero pixel output) on the screen even though the output should represent a dark pixel. A "hot line" is a vertical or horizontal line of white pixels. A "dead pixel" is a pixel output that is always low because there is a maximum current output (i.e., fully stored charges at the capacitor), so that it would always appear to be a dead pixel (i.e., maximum level pixel output) on the screen even though the output should represent a bright pixel. A "dead line" is a vertical or horizontal line of dead pixels. The white pixels and hot lines, or dead pixels and dead lines, are produced because the voltage level at the sensor 13 is about the same as the voltage level in the substrate 11. When the substrate 11 is biased to GROUND voltage, white pixels and hot lines are produced. On the other hand, when the substrate 11 is biased to V.sub.DD voltage, dead pixels and dead lines are produced. White pixels, hot lines, dead pixels and dead lines can severely affect the accuracy of the output of a sensor device 10.
One attempt to address and solve the current leakage problem is to re-design the circuit. Unfortunately, this attempt will introduce complexity into the circuit design, thereby increasing costs, and may even impact the performance of the sensor device 10.
As a result, there still remains a need for sensor device that minimizes the leakage of current from the sensor, while at the same maintaining a high level of accuracy and performance.